Now this is interesting to me. How bad do you have to feel before you begin to make these judgements? I am still relatively new to this whole weightlifting world, so I am not sure about my own reactions to training.

I know some days I feel like hammered shit, I can't even do my normal activities without a serious hitch in my giddyup, and there's no chance I can workout, but I go anyway and I manage to do some decent things, even if they are less weight than the week before. I also know for certain that some days I wake up stiff as a board and truly wonder if I'm going to make it through my session at all, much less my day, but then start to feel decent once I start lifting and get going. Then other days, I feel pretty sore, but fairly decent overall, and yet when I start to workout, I can't really do anything anywhere close to decent weight.

So, what's the line? How much soreness is acceptable? Or is it all mental? If your brain isn't gung ho, then you call it and move on?

This is something that you're gonna have to figure out for yourself my friend. Everyone is different with different tolerances to soreness and fatigue. When in doubt, take time off.

I'm pretty sure just about everyone who lifts wakes up stiff as a board, but that doesn't necessarily mean anything more than you should stretch first thing in the AM. If you're going into your workout feeling shaky and all is well, then you're probably fine. However, one day you will go in and things won't be so right, and that's your body's way of telling you its time for a break. That may be significant soreness carried over from the previous workout, not hitting typical numbers on multiple exercises, feeling tired for days on end, etc..

Having your numbers start to go in reverse over several workouts (2-3, everyone has off days) is a sure give away. If it happens, take time off or end up injured.

Again, this is all individualistic. We, and you, won't have the answer for everything; a lot of our success if built around trial and error and what we learn about ourselves along the way. You're just gonna have to find yours.